The basic principles underlying the structure and operation of one type of high-speed ferroelectric liquid crystal display device are described by Clark, et al., Applied Physics Letters, 36 (11) 1980. The display device of Clark, et al. includes a liquid crystal cell which employs a chiral smectic C liquid crystal material. A liquid crystal material of this type is characterized as having molecules that are arranged in parallel planes and tilted at acute angles relative to the normal to such planes. The molecules are rotatable about the normal to the planes and have permanent electric dipole moments that are perpendicular to the longitudinal axes of the molecules.
The liquid crystal cell of Clark, et al. includes many smectic planes which are disposed between and perpendicular to an opposed pair of parallel transparent electrode plates. Potential differences applied between the electrode plates generate in the region of the liquid crystal material electric fields that are directed toward one of the two plates. The electric fields couple with the dipole moments of the liquid crystal molecules such that the molecules assume one of at least two orientations that depend upon the directions of the electric fields. When such a liquid crystal cell is placed between a pair of neutral density linear polarizers whose transmission axes are orthogonally aligned, the molecular orientations are optically distinguishable, thereby forming a ferroelectric liquid crystal display device having a light transmitting state and an opaque state.
The liquid crystal cell of Clark, et al. employs a liquid crystal material that exhibits in consecutive order the isotropic, smectic A, and smectic C phases at successively lower temperatures. Such a liquid crystal material in the isotropic phase is introduced between the pair of electrode plates and then cooled until it enters the smectic A phase. The planar arrangement of the liquid crystal molecules is obtained by repeatedly shearing the electrode plates relative to each other until the smectic planes are formed. The liquid crystal molecules contacting the electrode structures lie substantially parallel to them. The device is then cooled to the smectic C phase. A principle problem of the shearing method of alignment is the difficulty associated with the manufacturability of a device of this type.
Another method of aligning the smectic planes entails the use of a rubbed polymer alignment layer such as that described by Patel, "A Reliable Method of Alignment for Smectic Liquid Crystals" Ferroelectrics, Vol 59, p. 137, 1984. The problem with a liquid crystal cell of this type is that it often exhibits smectic plane alignment defects after the cell has been switched between the light transmission and opaque states. The smectic plane alignment defects appear as zig-zag patterns on the surface of the liquid crystal cell.
Smectic C liquid crystal cells purportedly having relatively few smectic plane alignment defects are described by Uemura, et al., Alignment of Chiral Smectic C Liquid Crystal by Oblique Evapolation [sic] Method, Japan Display '86, 464-467. Uemura, et al. employ a liquid crystal material that exhibits in consecutive order the isotropic, nematic, smectic A, and smectic C phases at successively lower temperatures. Such a liquid crystal material in the isotropic phase is positioned between a pair of opposed electrode plates which have been treated so that the directors of the liquid crystal material in contact with the electrode plates lie at pretilt angles of between 20.degree. and 25.degree., the pretilt angles of the directors in contact with the two electrode plates being defined in the same rotational sense. Such a director configuration is sometimes called "anti-parallel alignment." The liquid crystal cell is then cooled so that the liquid crystal material successively enters the nematic, smectic A, and smectic C phases. Although the liquid crystal cells of Uemura, et al. are purportedly relatively free from smectic plane alignment defects, display devices incorporating such cells typically have undesirably narrow viewing angles.
The inventor of the liquid crystal cell described herein has experienced difficulty in obtaining uniform director alignment in large area liquid crystal cells employing anti-parallel director alignment. Display devices using such large area cells suffer from light leakage at various locations on the viewing surface because the nonuniform director alignment makes it impossible to align the polarization axes to achieve complete extinction.